There are various forms of touch sensitive controls which use a capacitive sensor to sense the presence of a body such as a user's finger. A touch sensitive capacitive sensor for example is disclosed in WO-97/23738 [1]. The capacitive sensing device disclosed in WO-97/23738 [1] relies on measuring the capacitance of a sensing electrode to a system reference potential (earth). A single coupling plate is provided and disposed to form a touch sensitive switch. The coupling plate is referred to as a key. In accordance with this example, the key is charged using a drive circuit for a drive part of a measurement cycle and then this charge is measured by transferring the induced charge from the key by a charge detection circuit during a measurement part of the cycle. Typically, a burst of measurement cycles are used to generate a plurality of signal sample values. The sensor can detect the presence of an object near the key as a result of a change in an amount of the charge induced onto the key. Effectively, this provides a measure of a change in capacitance of the key as a result of the presence of the body or object. When a pointing object for example a user's finger approaches the sensing electrode (plate), the pointing object appears to be a virtual ground. This serves to increase the measured capacitance of the sensing electrode to ground. Thus an increase in measured capacitance is taken to indicate the presence of a pointing object. The contents of WO-97/23738 [1] and corresponding U.S. Pat. No. 5,730,165 [3] are incorporated herein in their entirety by reference as background material to the invention.
Another form of touch sensitive control is disclosed in WO-00/44018 [2]. In this example a pair of electrodes are provided which act as a key so that the presence of a body such as a user's finger is detected as a result of a change in an amount of charge which is transferred between the two electrodes. With this arrangement, one of the electrodes (labelled X) is driven with a drive circuit and the other of the pair of electrodes (labelled Y) is connected to a charge measurement circuit which detects an amount of charge present on the Y plate when driven by the X plate. As disclosed in WO-00/440018 [2] several pairs of electrodes can be arranged to form a matrix of sensing areas which can provide an efficient implementation of a touch sensitive two-dimensional position sensor. Such two dimensional capacitive transducing (2DCT) sensors are typically used with devices which include touch sensitive screens or touch sensitive keyboards/keypads which are used in for example in consumer electronic devices and domestic appliances.
Example devices use 2DCT sensors in conjunction with an underlying display such as a liquid crystal display (LCD) or a cathode ray tube (CRT). Devices employing 2DCT sensors have become increasingly popular and common not only in conjunction with personal computers but also in all manner of other appliances such as personal digital assistants (PDAs), point of sale (POS) terminals, electronic information and ticketing kiosks, kitchen appliances and the like. 2DCT sensors are frequently preferred to mechanical switches for a number of reasons. For example, 2DCT sensors require no moving parts and so are less prone to wear than their mechanical counterparts. 2DCT sensors can also be made in relatively small sizes so that correspondingly small, and tightly packed keypad arrays can be provided. Furthermore, 2DCT sensors can be provided beneath an environmentally sealed outer surface/cover panel. This makes their use in wet environments or where there is a danger of dirt or fluids entering a device being controlled attractive. In addition, manufactures often prefer to employ interfaces based on 2DCT sensors in their products because such interfaces are often considered by consumers to be more aesthetically pleasing than conventional mechanical input mechanisms (e.g. push-buttons).
Other devices which may incorporate 2DCT sensors include pen-input tablets and encoders used in machinery for feedback control purposes, for example 2DCT sensors are capable of reporting at least a 2-dimensional coordinate, Cartesian or otherwise, related to the location of an object or human body part by means of a capacitance sensing mechanism.
U.S. Patent Application 60/803,510 [4] describes a 2DCT sensor comprising a substrate with a sensing area defined by a pattern of electrodes in which the electrodes are sensed by passive sensing techniques. A capacitance measurement circuit of the type described in the applicant's U.S. Pat. No. 6,288;707 [5] for example as shown in FIG. 5 of U.S. patent application 60/803,510 [4] is coupled to the sensing electrodes for determining a change in capacitance caused by the approach of a user's finger or other object to the sensing electrodes. Further details of sensor circuitry and methods of driving the sensor circuitry are available in U.S. Pat. No. 5,730,165 [3] and U.S. Pat. No. 7,148,704 [6]. However, it has been found that there are some limitations associated with 2DCT sensors which operate on passive sensing techniques. For example, passive 2DCT sensors can be sensitive to external ground loading. Furthermore, electrical noise generated from such LCD screens can interfere with capacitance measurements when a pointing object approaches the screen. Known methods to minimise the effects of noise on capacitive coupling is to incorporate a shielding layer between the LCD screen and a 2DCT sensor to reduce or block the noise induced by the LCD screen. However, such methods increase the size and thickness of a device incorporating a display screen with a 2DCT sensor when it is more fashionable and desirable to produce smaller devices. Furthermore; additional steps are required during manufacture and as a result there is an increased cost due to further components being needed.
European patent EP 1 821 175 [7] discloses a display device with a touch sensor which is arranged so that a two dimensional touch sensor is overlaid upon a display device to form a touch sensitive display screen. The display device uses an LCD arrangement with vertical and horizontal switching of the LCD pixels. The touch sensing circuit includes a current detection circuit, a noise elimination circuit as well as a sampling circuit for each of a plurality of sensors, which are arranged to form the two-dimensional sensor array. The current detection circuit receives a strobe signal, which is generated from the horizontal and vertical switching signals of the LCD screen. The strobe signal is used to trigger a blanking of the current detection circuit during a period in which the horizontal switching voltage signal may affect the measurements performed by the detection circuit.
Generally, a technical problem subsists in the implementation of touch sensors to reduce the effects of noise signals, which may be induced from another source on a sensing element of the touch sensor.